The invention relates to an inner tank, of flat design, which is surrounded by an outer tank and serves to hold a cryogenic liquid, in particular a fuel, in particular an inner tank for a road-going vehicle.
Usually, for use in motor vehicles, cryogenic fuels are inserted into cylindrical, that is to say barrel-shaped tanks which are designed as double-walled steel tanks. However, only poor space utilization is possible in this way; steel tanks of said type take up the entire luggage space of a motor vehicle. DE 101 63 029 A1 presents a barrel-shaped high-pressure hydrogen tank of said type in which an inner polyethylene lining is surrounded by an outer winding which is intended to prevent a bulging effect.
A flat, cuboidal design of tanks for cryogenic fuels is desirable in particular for use in motor vehicles. This creates the problem of obtaining sufficient strength and stability, in particular sufficient stiffness or rigidity against buckling, of the tank.
Conventional reinforcement measures for tanks with large planar surfaces are usually very massive. Thick sheet-metal wall thicknesses entail both complex production techniques and also joining techniques as a result of a large number of web plates or bulkhead plates used in the cryogenic inner tank; a high weight is the ultimate result.
A tank, of flat design, of the type described in the introduction is known for example from EP 1 067 300 A1. The tank has an upper shell and a lower shell composed of plastic, wherein the shells are connected to one another by means of a plurality of tubular tensile struts in order to prevent buckling of the tank as a result of the internal pressure. Although expedient space utilization is possible in this way, especially since the tank may be adapted to the available stowage space, it is however disadvantageous that a separate tank must be developed for each motor vehicle model, which in any case also always entails new dies and tools.
The invention is concerned with avoiding the disadvantages and difficulties associated with the prior art, and addresses the problem of creating a tank of the type described in the introduction, in particular a cryogenic tank for storing cooled liquid hydrogen, which can be produced in a simple and economical manner, which can be easily adapted during production to a predefined cuboidal installation space of a motor vehicle, which also has an expedient ratio of weight of the maximum liquid capacity to inherent weight, and which makes it possible to make do without subsequent machining, for example by providing bores for the purpose of filling and extraction.
The object is achieved by means of an inner tank wherein, to determine the bulging of the top wall and/or base wall, a planar reference top wall and/or reference base wall is defined, with said reference top wall and/or reference base wall being defined in each case as the planar connecting plane of the base-side or top-side straight ends of the webs.
According to one embodiment of the invention, the top wall and/or base wall has a curvature which is substantially dependent on the expected inner pressure in the inner tank. It is ensured according to the invention that, as a result of the predefined slight curvature, the occurring notch stresses between the curved surfaces of the base wall and/or of the top wall and also the effective dead spaces between the inner and outer tank are kept small.
Curvatures greater than the curvatures according to the invention pose a considerable risk to installation space efficiency, in particular since, according to the prior art, multi-layer isolation (MLI) should be provided around the inner tank.
According to one particular embodiment, one of the webs coincides with a substantially straight side wall. Here, specifically, a design of an inner tank having two chambers is provided, with a single web being provided between the two chambers, but at least one of the two side walls being of substantially straight design. In particular for the determination of the bulging of the base wall and/or top wall according to the claim, the side wall is considered here as a second web. The top wall and/or base wall preferably at least partially has a convex curvature as viewed from the outside.
According to one embodiment of the invention, the tank has the following features:                a longitudinally extending, single-piece basic body having a top wall and a base wall which are connected by means of likewise longitudinally extending side walls, and having at least one longitudinally running, substantially straight web which connects the base wall to the top wall so as to form at least two longitudinally extending chambers which are arranged adjacent to one another and which extend over the entire length of the basic body and in each case from the base wall to the top wall, and        caps which sealingly close off two open ends of the basic body in each case at the periphery and which engage over the ends of the chambers in each case, which caps form a free space which connects the chambers.        
According to one particular embodiment, at least one of the caps directly adjoins the basic body in an abutting fashion.
According to one embodiment of the invention, the top wall and/or base wall has, in each case with respect to a planar reference top wall and/or reference base wall, a bulge with a spacing, measured centrally between the webs, in each case between the inner contour of the top wall and/or base wall and the planar reference top wall and/or reference base wall, of less than 25%, preferably less than 20%, particularly preferably less than 15%, of the width of the chamber.
According to one embodiment of the invention, the base wall and/or the top wall are/is of substantially planar design.
According to one embodiment of the invention, the base wall and/or the top wall has, in each case with respect to a planar reference top wall and/or reference base wall, a bulge with a spacing, measured centrally between the webs, in each case between the inner contour of the top wall and/or base wall and the planar reference top wall and/or reference base wall, of more than 3%, in particular a spacing in the range between 5% and 15%, preferably between 5% and 10%, of the width of the chamber.
According to one embodiment of the invention, the caps engage over the ends of the chambers in each case. Here, according to various embodiments, provision is made of either individual caps which, at one end of the basic body, close off the chambers of the basic body separately, or of one cap which engages over a plurality of chambers and closes said chambers off together.
According to one embodiment of the invention, at least one of the caps forms a free space which connects the chambers. Said free space permits communication between the chambers of the basic body and it is possible in this way for an exchange or equalization of gas or liquid to take place between the chambers.
Alternatively, said exchange of gas or liquid could take place via corresponding openings in the webs in the region of the basic body.
According to one embodiment of the invention, at least one of the caps is of substantially flat or planar design. The production of such a cap is particularly simple and cost-effective. According to a further embodiment of the invention, a plurality of the caps are of substantially planar design, and at least one of the caps has a non-flat voluminous shape, with said cap being provided for the filling and/or extraction line and/or for holding the sensor arrangement, for example for determining the liquid level in the inner tank.
According to one embodiment of the invention, at least one of the webs has a cutout in the region of the fastening of at least one of the caps to the base wall and/or top wall. According to a preferred embodiment, all of the webs have at least one cutout, in particular in each case at each end. Here, a cutout is to be understood to mean in particular a recessing of an edge. As a result of the cutout in the region of the fastening, which is preferably produced according to the prior art by means of thermal processes such as welding or soldering processes, an excessive dissipation of heat via the web during the production of the fastening seam, in particular weld seam or solder seam, is prevented. In this way, it is possible to ensure a uniform fastening seam between the cap and the basic body. According to a further aspect, the cutouts can contribute to better communication of the chambers of the basic body, with it being possible for a corresponding exchange of gas or liquid between the chambers of the inner tank to take place through the cutouts.
According to one embodiment of the invention, at least one of the caps is fastened by means of a preferably encircling fastening seam, in particular a weld seam, to the basic body, and the webs have a cutout in each case in the region of the preferably encircling fastening seam, in particular weld seam.
According to one embodiment of the invention, at least one of the caps has a number of recesses into which the free end, which is aligned in the longitudinal direction of the basic body, of at least one of the webs projects. According to this embodiment, the stability of the inner tank and/or of the cap(s) which close(s) off the basic body can be increased significantly.
According to one embodiment, the web is fastened by means of a fastening seam, in particular a weld seam, to the cap in the region of the recess of the cap.
One embodiment, which is preferable with regard to strength and which is expedient in terms of production, is characterized in that the web is aligned approximately perpendicular to the base wall and to the top wall, which runs parallel to said base wall, of the basic body.
The inner tank preferably has a profile, transversely with respect to the longitudinal extent, in the shape of a rectangle, wherein for simple production of the caps, the longitudinal edges, which connect the side walls to the base wall and to the top wall, are of rounded design.
The basic body can be produced in a particularly expedient manner if it is designed as a single-piece extruded profile, with the wall thicknesses of all the walls preferably being approximately equal or the wall thickness of a web being smaller than that of the base wall, top wall and side walls.
The basic body can be produced in a particularly expedient manner if the wall thicknesses of all the walls are approximately equal or the wall thickness of a web is smaller than that of the base wall, top wall and side walls.
To avoid stress peaks, the inner tank is characterized in that the transitions between the web and base wall or top wall are of rounded design with a radius which is greater than or equal to the wall thickness of the web or the wall thickness of the base wall or top wall. According to a further embodiment, the transitions have a plurality of radii or at least portions of a free-form curve.
As a result of the high strength which can be obtained in structural terms, it is possible for the basic body and the caps to be formed from light metal, in particular from an aluminum alloy.
The caps are preferably deep-drawn, forged or cast, in particular by means of a pressure-die-casting process, with the caps expediently being connected to the basic body by means of an encircling weld seam.
The caps are preferably connected to the basic body by means of an encircling weld seam.
To connect the caps to the basic body, those edges which bear against one another expediently undergo pre-treatment, preferably at least one of the edges is provided with a rabbet so as to form an overlap of the components. A rabbet of said type is advantageous for the application of fusion welding processes.
Friction stir welding has proven to be particularly expedient for the production of the weld seam. It is also possible for so-called multi-orbital friction welding (“rotary friction welding” or “linear friction welding”) or so-called “friction stir welding” to be used for producing a weld seam.
According to one preferred embodiment, any tank fittings, such as sensors for pressure and/or temperature and/or filling level and the associated lines, are provided in the interior of a cap, and are if appropriate integrated in a pipe stub of a cap.
A filling opening and/or an extraction opening are/is preferably provided in one of the caps.
To secure the position of the inner tank in the outer tank, mounting elements are preferably arranged on the caps, which mounting elements mount the inner tank on the outer tank.
A mounting element is preferably designed as a mounting sleeve, preferably from an aluminum alloy and expediently in a local indentation of the cap, into which mounting sleeve a synthetic part, preferably a fiber composite part, can be inserted, which synthetic part projects into a mounting sleeve which is provided on the outer tank.
Furthermore, to secure the position of the inner tank with respect to the outer tank, it is additionally possible for spacers to be fastened to the caps or integrally formed with the caps.
The position of the inner tank is preferably further secured if a filling and extracting line is guided on the top wall on the outside of the inner tank.
According to the invention, an inner tank (3), of flat design, which is surrounded by an outer tank and serves to hold a cryogenic liquid, in particular a fuel, in particular an inner tank for a road-going vehicle, is provided, which inner tank is characterized by the combination of the following features:                a longitudinally extending, single-piece basic body having a top wall and a base wall which are connected by means of likewise longitudinally extending side walls, and having at least one longitudinally running, substantially straight web which connects the base wall to the top wall so as to form two longitudinally extending chambers which are arranged at both sides of the web and which extend over the entire length of the basic body and in each case from the base wall to the top wall, with the longitudinally extending chamber having a predetermined width between the webs, and        at least two caps which sealingly close off the two open ends of the basic body in each case at the periphery, and        with the web having a cutout in the region of the fastening of at least one of the caps to the base wall and/or top wall.        
According to one embodiment, at least one of the caps is fastened by means of a preferably encircling fastening seam, in particular a weld seam, to the basic body, and the webs have a cutout in each case in the region of the preferably encircling fastening seam, in particular weld seam.
According to one embodiment, at least one of the caps has a number of recesses into which the free end, which is aligned in the longitudinal direction of the basic body, of at least one of the webs projects.
According to one embodiment, the web is fastened by means of a fastening seam, in particular a weld seam, to the cap in the region of the recess of the cap.
According to the invention, an inner tank, of flat design, which is surrounded by an outer tank and serves to hold a cryogenic liquid, in particular a fuel, in particular an inner tank for a road-going vehicle, is provided, which inner tank is characterized by the combination of the following features:                a longitudinally extending basic body having a top wall and a base wall which are connected by means of likewise longitudinally extending side walls, and having at least one longitudinally running, substantially straight web which connects the base wall to the top wall so as to form two longitudinally extending chambers which are arranged at both sides of the web and which extend over the entire length of the basic body and in each case from the base wall to the top wall, and        at least two caps which sealingly close off the two open ends of the basic body in each case at the periphery, and        with at least one of the caps being of substantially flat design.        
According to one embodiment, the web has a cutout in the region of the fastening of at least one of the caps to the base wall and/or top wall.
According to one embodiment, at least one of the caps is fastened by means of a preferably encircling fastening seam, in particular a weld seam, to the basic body, and the web has a cutout in each case in the region of the preferably encircling fastening seam, in particular weld seam.
According to one embodiment, at least one of the caps has a number of recesses into which the free end, which is aligned in the longitudinal direction of the basic body, of the web projects.
According to one embodiment, the web is fastened by means of a fastening seam, in particular a weld seam, to the cap in the region of the recess of the cap.